Liquid crystal display device

ABSTRACT

According to one embodiment, a liquid crystal display device includes a first insulative substrate, a second insulative substrate, a first polarizer disposed on an outer surface side of the first insulative substrate, a second polarizer disposed on an outer surface side of the second insulative substrate, and a first retardation plate and a second retardation plate, which are stacked between the second insulative substrate and the second polarizer. When a retardation in the thickness direction, which is defined by ((nx+ny)/2−nz)*d, is Rth, the first retardation plate has a negative first retardation Rth 1 , the second retardation plate has a positive second retardation Rth 2 , and a contribution degree of the second retardation Rth 2 , which is defined by |Rth 2 |*100/(|Rth 1 |+|Rth 2 |), is 40%±3%.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromprior Japanese Patent Application No. 2010-165210, filed Jul. 22, 2010,the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a liquid crystaldisplay device.

BACKGROUND

By virtue of such features as light weight, small thickness and lowpower consumption, liquid crystal display devices have been used invarious fields as display devices of OA equipment such as a personalcomputer, a TV, etc. In recent years, liquid crystal display deviceshave also been used as display devices of a portable terminal devicesuch as a mobile phone, a car navigation apparatus, a game machine, etc.

In such liquid crystal display devices, in some cases, retardation filmsare disposed to optically compensate retardation due to birefringence ofa liquid crystal layer in a frontal direction and an oblique direction.As such retardation films, there have been proposed retardation films inwhich, when the refractive indices in an in-plane slow axis direction,an in-plane fast axis direction and a thickness direction are nx, ny andnz, respectively, the in-plane retardation value (also referred to as“in-plane retardation Re” in this specification) defined by (nx−ny)*d orthe thickness-directional retardation value (also referred to as“retardation Rth in the thickness direction” in this specification)defined by ((nx+ny)/2−nz)*d is set in various ranges.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view which schematically shows the structure of aliquid crystal display device according to an embodiment.

FIG. 2 is a view which schematically shows the structure and anequivalent circuit of a liquid crystal display panel shown in FIG. 1.

FIG. 3 is a schematic plan view showing an example of the structure of apixel in an array substrate shown in FIG. 2, as viewed from acounter-substrate side.

FIG. 4 is a view which schematically shows a cross-sectional structureof the liquid crystal display panel, illustrating a cross section of thepixel along line A-B in FIG. 3.

FIG. 5 is a view for explaining the relationship between the angles ofaxes of various optical elements which are applied in the embodiment,FIG. 5 being a top view at a time when the liquid crystal display panelis observed from the counter-substrate side.

FIG. 6 is a view for explaining the principle of operation in the liquidcrystal display device of the embodiment.

FIG. 7 is a view for explaining the characteristics of a firstretardation plate and a second retardation plate which are applicable inthe embodiment.

FIG. 8 is a view showing a simulation result of the viewing anglecharacteristics of the contrast ratio in a case where the firstretardation plate and second retardation plate shown in FIG. 7 areapplied to the liquid crystal display device of the embodiment.

FIG. 9 is a view showing a simulation result of the viewing anglecharacteristics of the gray level inversion in a case where the firstretardation plate and second retardation plate shown in FIG. 7 areapplied to the liquid crystal display device of the embodiment.

FIG. 10 is a view for explaining the characteristics of firstretardation plates and second retardation plates which are applicable toModifications 1, 2 and 3 of the embodiment, Comparative Example 1 inwhich neither the first retardation plate nor second retardation plateis applied, and the characteristics of a first retardation plate and asecond retardation plate of Comparative Example 2.

FIG. 11 shows a simulation result of viewing angle characteristics ofthe contrast ratio in a liquid crystal display device of ComparativeExample 1.

FIG. 12 shows a simulation result of viewing angle characteristics ofthe gray level inversion in the liquid crystal display device ofComparative Example 1.

FIG. 13 shows a simulation result of viewing angle characteristics ofthe contrast ratio in a liquid crystal display device of ComparativeExample 2.

FIG. 14 shows a simulation result of viewing angle characteristics ofthe gray level inversion in the liquid crystal display device ofComparative Example 2.

DETAILED DESCRIPTION

In general, according to one embodiment, a liquid crystal display devicecomprises a first substrate including a first insulative substrate, acommon electrode disposed on an inner surface side of the firstinsulative substrate, an insulation film covering the common electrode,a pixel electrode which is disposed above the insulation film, opposedto the common electrode and provided with a slit, and a first alignmentfilm covering the pixel electrode; a second substrate including a secondinsulative substrate and a second alignment film disposed on an innersurface side of the second insulative substrate, the inner surface sideof the second insulative substrate being opposed to the pixel electrode;a liquid crystal layer held between the first substrate and the secondsubstrate; a first polarizer disposed on an outer surface side of thefirst insulative substrate; a second polarizer disposed on an outersurface side of the second insulative substrate; and a first retardationplate and a second retardation plate, which are stacked between thesecond insulative substrate and the second polarizer, wherein, withrespect to the first retardation plate and the second retardation plate,when refractive indices in two directions, which are perpendicular toeach other in a plane of each of the first retardation plate and thesecond retardation plate, are nx and ny, respectively, a refractiveindex in a thickness direction is nz, a thickness is d, and aretardation in the thickness direction, which is defined by((nx+ny)/2−nz)*d, is Rth, the first retardation plate has a negativefirst retardation Rth1, the second retardation plate has a positivesecond retardation Rth2, a sum of the first retardation Rth1 and thesecond retardation Rth2 is −40 nm±20 nm with respect to light with awavelength of 550 nm, and a contribution degree of the secondretardation Rth2, which is defined by |Rth2|*100/(|Rth1|+|Rth2|), is40%±3%.

According to another embodiment, a liquid crystal display devicecomprises a first substrate including a first insulative substrate, acommon electrode disposed on an inner surface side of the firstinsulative substrate, an insulation film covering the common electrode,a pixel electrode which is disposed above the insulation film, opposedto the common electrode and provided with a slit, and a first alignmentfilm covering the pixel electrode; a second substrate including a secondinsulative substrate and a second alignment film disposed on an innersurface side of the second insulative substrate, the inner surface sideof the second insulative substrate being opposed to the pixel electrode;a liquid crystal layer held between the first substrate and the secondsubstrate; a first polarizer disposed on an outer surface side of thefirst insulative substrate; a second polarizer disposed on an outersurface side of the second insulative substrate; and a first retardationplate and a second retardation plate, which are stacked between thesecond insulative substrate and the second polarizer, wherein, withrespect to the first retardation plate and the second retardation plate,when refractive indices in two directions, which are perpendicular toeach other in a plane of each of the first retardation plate and thesecond retardation plate, are nx and ny, respectively, a refractiveindex in a thickness direction is nz, and a thickness is d, the firstretardation plate has one of a refractive index anisotropy of nx=ny<nzand a refractive index anisotropy of nz>nx>ny, and the secondretardation plate has a refractive index anisotropy of nx>ny>nz, thefirst retardation plate has a first retardation Rth1 and the secondretardation plate has a second retardation Rth2 when a retardation inthe thickness direction, which is defined by ((nx+ny)/2−nz)*d, is Rth,and a contribution degree of the second retardation Rth2, which isdefined by |Rth2|*100/(|Rth1|+|Rth2|), is 40%±3%.

According to another embodiment, a liquid crystal display devicecomprises a first substrate including a first insulative substrate, acommon electrode disposed on an inner surface side of the firstinsulative substrate, an insulation film covering the common electrode,a pixel electrode which is disposed above the insulation film, opposedto the common electrode and provided with a slit, and a first alignmentfilm covering the pixel electrode; a second substrate including a secondinsulative substrate and a second alignment film disposed on an innersurface side of the second insulative substrate, the inner surface sideof the second insulative substrate being opposed to the pixel electrode;a liquid crystal layer held between the first substrate and the secondsubstrate; a first polarizer disposed on an outer surface side of thefirst insulative substrate; a first retardation plate disposed on anouter surface of the second insulative substrate; a second retardationplate stacked on the first retardation plate; and a second polarizerstacked on the second retardation plate, wherein, with respect to thefirst retardation plate and the second retardation plate, whenrefractive indices in two directions, which are perpendicular to eachother in a plane of each of the first retardation plate and the secondretardation plate, are nx and ny, respectively, a refractive index in athickness direction is nz, and a thickness is d, a sum of in-planeretardations Re, which are defined by (nx−ny)*d, is 115 nm±15 nm withrespect to light with a wavelength of 550 nm, and a sum of retardationsRth in the thickness direction, which are defined by ((nx+ny)/2−nz)*d,is −40 nm±20 nm with respect to light with a wavelength of 550 nm.

Embodiments will now be described in detail with reference to theaccompanying drawings. In the drawings, structural elements having thesame or similar functions are denoted by like reference numerals, and anoverlapping description is omitted.

FIG. 1 is a plan view which schematically shows the structure of aliquid crystal display device 1 according to an embodiment.

Specifically, the liquid crystal display device 1 includes anactive-matrix-type liquid crystal display panel LPN. In the exampleillustrated, the liquid crystal display device 1 includes a driving ICchip 2 and a flexible wiring board 3 as signal sources which supplynecessary signals for displaying an image on the liquid crystal displaypanel LPN.

The liquid crystal display panel LPN includes an array substrate AR as afirst substrate, a counter-substrate CT as a second substrate which isdisposed to be opposed to the array substrate AR, and a liquid crystallayer which is disposed between the array substrate AR and thecounter-substrate CT. The array substrate AR and the counter-substrateCT are attached by a sealant SE. The liquid crystal layer is held in aninside surrounded by the sealant SE in a cell gap which is createdbetween the array substrate AR and the counter-substrate CT. The sealantSE is formed, for example, in a substantially rectangular frame shapebetween the array substrate AR and the counter-substrate CT, and forms aclosed loop.

The liquid crystal display panel LPN includes an active area ACT, whichdisplays an image, in the inside surrounded by the sealant SE. Theactive area ACT has a substantially rectangular shape and is composed ofa plurality of pixels PX which are arrayed in a matrix of m×n (m and nare positive integers). The driving IC chip 2 and flexible wiring board3 are mounted on the array substrate AR in a peripheral area PRP on theoutside of the active area ACT.

FIG. 2 is a view which schematically shows the structure and anequivalent circuit of the liquid crystal display panel LPN shown inFIG. 1. A description is given of the structure in which the arraysubstrate AR of the liquid crystal display panel LPN includes a pixelelectrode PE and a common electrode CE, and a fringe field switching(FFS) mode is applied to switch liquid crystal molecules whichconstitute a liquid crystal layer LQ by mainly making use of atransverse electric field (i.e. an electric field substantially parallelto a major surface of the substrate) which is produced between the pixelelectrode PE and common electrode CE.

The array substrate AR includes, in the active area ACT, an n-number ofgate lines G (G1 to Gn), an n-number of capacitance lines C (C1 to Cn),an m-number of source lines S (S1 to Sm), an (m×n) number of switchingelements SW which are formed in respective pixels PX, an (m×n) number ofpixels PE which are formed in the respective pixels PX, and a commonelectrode CE which is formed common to the plural pixels PX.

The gate lines G and capacitance lines C extend in a first direction X.The source lines S extend in a second direction Y which is substantiallyperpendicular to the first direction X. The switching elements SW areelectrically connected to the gate lines G and source lines S. The pixelelectrode PE is electrically connected to the switching element SW ofeach pixel PX. The common electrode CE is a part of the capacitance lineC, and is opposed to the pixel electrode PE in each pixel PX. A storagecapacitance CS is formed, for example, between the capacitance line Cand pixel electrode PE. The liquid crystal layer LQ is interposedbetween the pixel electrode PE and the common electrode CE.

Each of the gate lines G is led out of the active area ACT and isconnected to a first driving circuit GD. Each of the source lines S isled out of the active area ACT and is connected to a second drivingcircuit SD. Each of the capacitance lines C is led out of the activearea ACT and is connected to a third driving circuit CD. The firstdriving circuit GD, second driving circuit SD and third driving circuitCD are formed on the array substrate AR and are connected to the drivingIC chip 2.

In the example illustrated, the driving IC chip 2 is mounted on thearray substrate AR on the outside of the active area ACT. The depictionof the flexible wiring board is omitted, and terminals T for connectionto the flexible wiring board are formed on the array substrate AR. Theseterminals T are connected to the driving IC chip 2 via various wiringlines.

FIG. 3 is a schematic plan view showing an example of the structure ofthe pixel PX in the array substrate AR shown in FIG. 2, as viewed fromthe counter-substrate CT side.

The gate lines G extend in a substantially linear shape along the firstdirection X. The source lines S extend in a substantially linear shapealong the second direction Y. The switching element SW is disposed at anintersection between the gate line G and source line S. The switchingelement SW is composed of, for example, a thin-film transistor (TFT).The switching element SW includes a semiconductor layer SC. Thesemiconductor layer SC can be formed of, for example, polysilicon oramorphous silicon. In this example, the semiconductor layer SC is formedof polysilicon.

A gate electrode WG of the switching element SW is located immediatelyabove the semiconductor layer SC, and is electrically connected to thegate line G (in the example illustrated, the gate electrode WG is formedintegral with the gate line G). A source electrode WS of the switchingelement SW is electrically connected to the source line S (in theexample illustrated, the source electrode WS is formed integral with thesource line S). A drain electrode WD of the switching element SW iselectrically connected to the pixel electrode PE.

The capacitance line C extends in the first direction X. The capacitanceline C includes the common electrode CE which is formed in a manner tocorrespond to the respective pixels PX, and extends over a part of thesource line S. The pixel electrode PE is disposed above the commonelectrode CE. The pixel electrode PE is formed in an island shapecorresponding to the pixel shape in the pixel PX, for example, in asubstantially rectangular shape.

Slit PSL are formed in the pixel electrode PE. In the exampleillustrated, the slits PSL extend in a linear shape in the seconddirection Y. Specifically, the direction of extension of the slits PSLis substantially parallel to the direction of extension of the sourcelines S. The slits PSL are formed above the common electrode CE. By suchslit shapes, single domains are formed in the respective pixels PX.

FIG. 4 is a view which schematically shows a cross-sectional structureof the liquid crystal display panel LPN, illustrating a cross section ofthe pixel PX along line A-B in FIG. 3.

Specifically, the array substrate AR is formed by using a firstinsulative substrate 20 having light transmissivity, such as a glasssubstrate. The array substrate AR includes the switching element SW,common electrode CE, pixel electrode PE and first alignment film 25 onan inner surface of the first insulative substrate 20 (i.e. the surfaceopposed to the counter-substrate CT). The switching element SW shown inFIG. 4 is a top-gate-type thin-film transistor. The semiconductor layerSC is formed on the first insulative substrate 20. The semiconductorlayer SC is covered with a gate insulation film 21. In addition, thegate insulation film 21 is also formed on the first insulative substrate20. In the meantime, an undercoat layer may be disposed as an insulationfilm between the first insulative substrate 20 and the semiconductorlayer SC.

The gate electrode WG of the switching element SW is formed on the gateinsulation film 21 and is located immediately above the semiconductorlayer SC. The gate electrode WG is covered with a first interlayerinsulation film 22. The first interlayer insulation film 22 is alsoformed on the gate insulation film 21. The gate insulation film 21 andfirst interlayer insulation film 22 are formed of an inorganic materialsuch as silicon nitride (SiN).

The source electrode WS and drain electrode WD of the switching elementSW are formed on the first interlayer insulation film 22. The sourceelectrode WS and drain electrode WD are put in contact with thesemiconductor layer SC via contact holes which penetrate the gateinsulation film 21 and first interlayer insulation film 22. In addition,the source line S is formed on the first interlayer insulation film 22.The gate electrode WG, source electrode WS and drain electrode WD areformed of an electrically conductive material such as molybdenum,aluminum, tungsten or titanium.

The source electrode WS and drain electrode WD are covered with a secondinterlayer insulation film 23. In addition, the second interlayerinsulation film 23 is formed on the first interlayer insulation film 22.The capacitance line C including the common electrode CE is formed onthe second interlayer insulation film 23, and extends over the sourceline S. The common electrode CE and capacitance line C are covered witha third interlayer insulation film 24. The third interlayer insulationfilm 24 is also formed on the second interlayer insulation film 23.

The pixel electrode PE is formed on the third interlayer insulation film24. The pixel electrode PE is connected to the drain electrode WD via acontact hole which penetrates the second interlayer insulation film 23and third interlayer insulation film 24. The slit PSL, which is opposedto the common electrode CE, is formed in the pixel electrode PE. Thepixel electrode PE is opposed to the common electrode CE via the secondinterlayer insulation film 24.

The common electrode CE, capacitance line C and pixel electrode PE areformed of a transparent, electrically conductive material, such as anindium tin oxide (ITO) or an indium zinc oxide (IZO). The pixelelectrode PE and second interlayer insulation film 24 are covered with afirst alignment film 25.

Specifically, the first alignment film 25 is disposed on that surface ofthe array substrate AR, which is in contact with the liquid crystallayer LQ.

On the other hand, the counter-substrate CT is formed by using a secondinsulative substrate 30 having light transmissivity, such as a glasssubstrate. The counter-substrate CT includes a black matrix 31, a colorfilter 32, an overcoat layer 33 and a second alignment film 34 on theinner surface of the second insulative substrate 30 (i.e. the surfaceopposed to the array substrate AR).

The black matrix 31 partitions the pixels PX. The black matrix 31 isformed on the second insulative substrate 30. To be more specific, inthe active area ACT, the black matrix 31 is formed to be positionedimmediately above the wiring portions such as the gate line G, sourceline S and switching element SW, which are provided on the arraysubstrate AR. The black matrix 31 is formed in a lattice shape or instripes. The black matrix 31 is formed of a light-blocking metallicmaterial such as a black-colored resin material or chromium (Cr).

The color filter 32 is formed in each pixel PX. The color filter 32 isformed on the second insulative substrate 30. To be more specific, inthe active area ACT, the color filter 32 includes a red color filterwhich is disposed to correspond to a red pixel, a blue color filterwhich is disposed to correspond to a blue pixel, and a green colorfilter which is disposed to correspond to a green pixel. A part of thecolor filter 32 is laid over the black matrix 31. The red color filter,blue color filter and green color filter are formed of, for example,resin materials which are colored in the respective colors.

In the above-described liquid crystal mode which makes use of thetransverse electric field, it is desirable that the surface of thecounter-substrate CT, which is in contact with the liquid crystal layerLQ, be planar. In the example illustrated, the overcoat layer 33 coversthe black matrix 31 and color filter 32 and planarizes the asperities onthe surfaces thereof. The overcoat layer 33 is formed of, for example, atransparent resin material.

The overcoat layer 33 is covered with the second alignment film 34.Specifically, the second alignment film 34 is disposed on that surfaceof the counter-substrate CT, which is in contact with the liquid crystallayer LQ. The first alignment film 25 and second alignment film 34 areformed of, for example, polyimide.

The above-described array substrate AR and counter-substrate CT aredisposed such that the first alignment film 25 and second alignment film34 face each other. In this case, spacers (e.g. columnar spacers whichare formed of resin material so as to be integral with the arraysubstrate AR), which are not shown, are disposed between the arraysubstrate AR and counter-substrate CT. Thereby, a predetermined cell gapis created between the array substrate AR and the counter-substrate CT.The array substrate AR and counter-substrate CT are attached by asealant SE in the state in which the predetermined cell gap is created.

The liquid crystal layer LQ is formed of a liquid crystal compositionwhich is sealed in the cell gap that is created between the firstalignment film 25 of the array substrate AR and the second alignmentfilm 34 of the counter-substrate CT.

A first polarizer PL1 is disposed on the side of the outer surface ofthe array substrate AR, that is, on the outer surface of the firstinsulative substrate 20. The first polarizer PL1 is fixed to the outersurface of the first insulative substrate 20 by means of, e.g. adhesion.

A second polarizer PL2 is disposed on the side of the other outersurface of the counter-substrate CT, that is, the outer surface of thesecond insulative substrate 30. A first retardation plate RP1 and asecond retardation plate RP2 are stacked between the second insulativesubstrate 30 and the second polarizer PL2. In the example illustrated,the first retardation plate RP1 is disposed on the outer surface of thesecond insulative substrate 30, the second retardation plate RP2 isstacked on the first retardation plate RP1, and the second polarizer PL2is stacked on the second retardation plate RP2.

The first retardation plate RP1, second retardation plate RP2 and secondpolarizer PL2 are formed as one set by means of, e.g. adhesion. The setof the first retardation plate RP1, second retardation plate RP2 andsecond polarizer PL2 is fixed such that the first retardation plate RP1,for example, is adhered to the outer surface of the second insulativesubstrate 30.

FIG. 5 is a view for explaining the relationship between the angles ofaxes of various optical elements which are applied in the embodiment,FIG. 5 being a top view at a time when the liquid crystal display panelLPN is observed from the counter-substrate CT side, that is, from above.

In the liquid crystal display panel LPN, the slits PSL, which are formedin each pixel electrode PE, extend in the second direction Y, asdescribed above. The first alignment film 25 of the array substrate ARis subjected to alignment treatment in a θ direction. Specifically, afirst alignment treatment direction R1 of the first alignment film 25 isthe θ direction. The direction of extension of the slit PSL (the seconddirection Y) is a direction slightly inclined counterclockwise relativeto the θ direction. In this example, the direction of the slit PSLcorresponds to a direction which is displaced counterclockwise by 5° to10° relative to the θ direction. In other words, the θ direction is adirection which is displaced clockwise by 5° to 10° from the seconddirection Y.

The second alignment film 34 of the counter-substrate CT is subjected toalignment treatment in a (θ+180°) direction. Specifically, a secondalignment treatment direction R2 of the second alignment film 34 isparallel to, and opposite to, the first alignment treatment direction R1of the first alignment film 25. The first alignment treatment directionR1 of the first alignment film 25 and the second alignment treatmentdirection R2 of the second alignment film 34 are the directions in whichrubbing treatment and optical alignment treatment are performed.

The first polarizer PL1 has a first absorption axis A1 which issubstantially parallel to the first alignment treatment direction R1 ofthe first alignment film 25. Specifically, the first absorption axis A1is parallel to the θ direction. In some cases, the first absorption axisis referred to as “first polarization axis”. The second polarizer PL2has a second absorption axis A2 which is substantially perpendicular tothe first alignment treatment direction R1 or the first absorption axisA1. Specifically, the second absorption axis A2 is parallel to a (θ+90°)direction. In some cases, the second absorption axis is referred to as“second polarization axis”.

The first retardation plate RP1 and second retardation plate RP2 aredisposed over the entirety of the active area ACT. The details of thefirst retardation plate RP1 and second retardation plate RP2 will bedescribed later.

FIG. 6 is a view for explaining the principle of operation in the liquidcrystal display device of the embodiment. A description is given withreference to the cross-section of the liquid crystal display panel LPNin the (θ+90°) direction that is parallel to the second absorption axisA2. Only the structure that is necessary for the description isillustrated.

A left part of FIG. 6 illustrates an OFF state in which no electricfield is produced between the pixel electrode PE and the commonelectrode CE in the liquid crystal display device of the presentembodiment. A right part of FIG. 6 illustrates an ON state in which anelectric field (fringe electric field) is produced between the pixelelectrode PE and the common electrode CE via the slit PSL. The liquidcrystal display device shown in FIG. 6 operates in a linear polarizationmode.

In the OFF state, the axis of alignment of liquid crystal molecules LM,which are homogenously aligned in the liquid crystal layer LQ in thefirst alignment treatment direction R1 of the first alignment film andthe second alignment treatment direction R2 of the second alignmentfilm, is parallel to the first absorption axis A1 of the first polarizerPL1 and is perpendicular to the second absorption axis A2 of the secondpolarizer PL2. In the OFF state, linearly polarized light, which isemitted from a backlight BL and passes through the first polarizer PL1,travels through the liquid crystal display panel LPN and is thenabsorbed in the second polarizer PL2. Thus, black display is effected inthe OFF state.

On the other hand, in the ON state, some of the liquid crystal moleculesLM are affected by the fringe electric field and the alignment statethereof changes. At this time, the axis of alignment of liquid crystalmolecules LM is displaced from the first absorption axis A1 of the firstpolarizer PL1 and the second absorption axis A2 of the second polarizerPL2. In the ON state, the liquid crystal layer LQ has a retardation Δn·dwhich corresponds to a ½ wavelength (Δn is the refractive indexanisotropy of the liquid crystal layer LQ, and d is the thickness of theliquid crystal layer LQ). Thus, linearly polarized light, which passesthrough the first polarizer PL1, changes its polarization state due tothe effect of the retardation of the liquid crystal layer LQ whiletraveling through the liquid crystal display panel LPN, and the lightemerging from the liquid crystal display panel LPN passes through thesecond polarizer PL2. Accordingly, white display is effected. Thereby, anormally black mode is realized.

As described above, in the ON state, the liquid crystal layer LQ has aretardation Δn·d which corresponds to a ½ wavelength of the wavelength λof transmissive light. Meanwhile, in the OFF state, the liquid crystallayer LQ has a retardation Δn·d which is higher than the ½ wavelength,and has a retardation Δn·d of 300 nm to 350 nm, relative to light with awavelength of 550 nm.

In each of the OFF state and ON state, the light, which has passedthrough the liquid crystal display panel LPN, is affected by a properretardation when the light passes through the first retardation plateRP1 and second retardation plate RP2. One of the retardation platesmainly has such a refractive index anisotropy that the refractive indexin the thickness direction is relatively high, with respect to theliquid crystal molecules LM which are homogenously aligned substantiallyin parallel to the major surface of the substrate. Thereby, this oneretardation plate executes optical compensation so as to impartisotropic optical characteristics to the liquid crystal molecules LM,regardless of the viewing angle. The other retardation plate executesoptical compensation so that the first polarization axis and secondpolarization axis, which are in a positional relationship of crossedNicols, may keep the same positional relationship regardless of theviewing angle.

Thereby, the light, which has passed through the liquid crystal displaypanel LPN, is optically compensated so as to exhibit equal opticalcharacteristics in a frontal direction which is parallel to the normalline of the liquid crystal display panel LPN or in an oblique directionwhich is inclined to the normal line. Hence, the range of the viewingangle, at which observation with an equal contrast ratio (CR) ispossible, can be increased, and the range of the viewing angle, at whichno gray level inversion is observed, can be increased.

Next, a description is given of the first retardation plate RP1 andsecond retardation plate RP2, which are applicable to the presentembodiment.

In the present embodiment, when various characteristics of theretardation plates are discussed, the refractive indices in twodirections, which are perpendicular to each other in the plane of theretardation plate, are nx and ny, the refractive index in the thicknessdirection is nz, and the thickness of each retardation plate is d. Inthis case, nx corresponds to the refractive index in the slow axisdirection in the plane, and ny corresponds to the refractive index inthe fast axis direction in the plane.

In the retardation plate, the retardation Re in the plane is defined bythe following equation:Re=(nx−ny)*d.

The first retardation plate RP1 has a first retardation Re1, and thesecond retardation plate RP2 has a second retardation Re2. There may bea case, however, in which the retardation Re1 and retardation Re2 arezero. In the present embodiment, it is desirable that the sum of thefirst retardation Re1 of the first retardation plate RP1 and the secondretardation Re2 of the second retardation RP2 (Re(sum)=Re1+Re2) be setwithin the range of 115 nm±15 nm.

In the retardation plate, the retardation Rth in the thickness directionis defined by the following equation:Rth=((nx+ny)/2−nz)*d.

The first retardation plate RP1 has a first retardation Rth1, and thesecond retardation plate RP2 has a second retardation Rth2. In thepresent embodiment, it is desirable that the sum of the firstretardation Rth1 of the first retardation plate RP1 and the secondretardation Rth2 of the second retardation RP2 (Rth(sum)=Rth1+Rth2) beset within the range of −40 nm±20 nm.

The values of both retardations Re and Rth described here are valueswith respect to light with a wavelength of 550 nm.

Either the first retardation plate RP1 or the second retardation plateRP2 has a refractive index anisotropy of nx>ny>nz. The optical axis inthe plane of the retardation plate having this refractive indexanisotropy is substantially perpendicular to the second absorption axisA2 of the second polarizer PL2, or is substantially parallel to thefirst alignment treatment direction R1 and second alignment treatmentdirection R2.

Next, concrete examples of the above are described.

FIG. 7 is a view for explaining the characteristics of the firstretardation plate RP1 and second retardation plate RP2 which areapplicable in the embodiment.

The first retardation plate RP1 has a refractive index anisotropy ofnx=ny<nz. The first retardation plate RP1 is formed of, for example, aliquid crystal polymer. In the first retardation plate RP1, the value ofthe first retardation Re1 is 0 nm, and the value of the firstretardation Rth1 is −150 nm.

The second retardation plate RP2 has a refractive index anisotropy ofnx>ny>nz. The second retardation plate RP2 is formed of a materialdifferent from the material of the first retardation plate RP1, forexample, cycloolefin polymer. In the second retardation plate RP2, thevalue of the second retardation Re2 is 115 nm, and the value of thesecond retardation Rth2 is 103.5 nm. The optical axis O in the plane ofthe second retardation plate RP2 is substantially perpendicular to thesecond absorption axis A2 of the second polarizer plate PL2 (see FIG.5).

Accordingly, as regards the first retardation plate RP1 and secondretardation plate RP2, the sum Re(sum) of the retardations Re is 115 nm,and the sum Rth(sum) of the retardations Rth is −46.5 nm. One aspect ofthe present embodiment is that the sum Re(sum) of the retardations Re inthe first retardation plate RP1 and second retardation plate RP2 is inthe range of 100 nm to 130 nm, or that the sum Rth(sum) of theretardations Rth is in the range of −60 nm to −20 nm.

Meanwhile, the first retardation plate RP1 and second retardation plateRP2 are formed of different materials. When a desired retardation Re orretardation Rth is to be obtained by combining them, it is necessary toconsider the wavelength dispersion characteristics of the respectiveretardation plates. Specifically, the first retardation plate RP1 has awavelength dispersion which is different from the wavelength dispersionof the second retardation plate RP2. In addition, as regards an Nzcoefficient which is defined by Nz=(nx−nz)/(nx−ny), the firstretardation plate RP1 has an Nz coefficient which is different from theNz coefficient of the second retardation plate RP2. Thus, even in thecase where the desired retardation is to be obtained, it is notsufficient that the sum of the retardations merely falls within theabove-described range. It is necessary to consider the degree ofcontribution as an index which indicates how the retardation of one ofthe retardation plates contributes to the sum of the retardations of thetwo retardation plates. In particular, the value of the retardation Rthof one of the retardation plates is a negative value, and the value ofthe retardation Rth of the other retardation plate is a positive value.In this description, the degree of contribution of the retardation platehaving a positive retardation Rth, or the retardation plate having arefractive index anisotropy of nx>ny>nz, is defined. The secondretardation plate RP2 in the example shown in FIG. 7 corresponds to theretardation plate having a positive second retardation Rth2 (or theretardation plate having a refractive index anisotropy of nx>ny>nz). Thecontribution degree CNT (%) of the second retardation Rth2 is defined bythe following equation:CNT2(%)=|Rth2|*100/(|Rth1|+|Rth2|).

Similarly, the contribution degree CNT (%) of the first retardation Rth1is defined by the following equation:CNT1(%)=|Rth1|*100/(|Rth1|+|Rth2|).

Another aspect of the present embodiment is that the contribution degreeCNT2 of the second retardation Rth2 is set in the range of 40%±3%. Inthe example shown in FIG. 7, the contribution degree CNT2 of the secondretardation Rth2 is 40.8%. In other words, the contribution degree CNT1of the first retardation Rth1 should desirably be set in the range of60%±3%, and is 59.2% in the above example.

FIG. 8 is a view showing a simulation result of the viewing anglecharacteristics of the contrast ratio in a case where the firstretardation plate RP1 and second retardation plate RP2 shown in FIG. 7are applied to the liquid crystal display device of the presentembodiment.

In FIG. 8, the center corresponds to a direction parallel to the normalline of the liquid crystal display panel LPN. Concentric circles aboutthe normal line correspond to positions with tilt angles (viewingangles) of 10°, 20°, 30°, 40°, 50°, 60°, 70° and 80° to the normal line.In addition, in FIG. 8, a 0° azimuth direction corresponds to a positiveside (+) in the X direction in which the above-described gate linesextend, and a 180° azimuth direction corresponds to a negative side (−)in the X direction. A 90° azimuth direction corresponds to a positiveside (+) in the Y direction in which the above-described source linesextend, and a 270° azimuth direction corresponds to a negative side (−)in the Y direction. Referring to similar Figures, a description will begiven later of simulation results of the viewing angle characteristicsof the contrast ratio and the viewing angle characteristics of the graylevel inversion.

As shown in FIG. 8, according to the present embodiment, it wasconfirmed that the contrast ratio (CR) is 10:1 or more in all azimuthdirections. As regards regions where the contrast ratio is 100:1 ormore, it was confirmed that the following ranges of viewing angles canbe obtained: about 60° at 0° azimuth, about 55° at 45° azimuth, about80° at 90° azimuth, about 50° at 135° azimuth, about 60° at 180°azimuth, about 60° at 225° azimuth, about 80° at 270° azimuth, and about50° at 315° azimuth.

FIG. 9 is a view showing a simulation result of the viewing anglecharacteristics of the gray level inversion in a case where the firstretardation plate RP1 and second retardation plate RP2 shown in FIG. 7are applied to the liquid crystal display device of the embodiment. Apart where a gray level inversion occurred corresponds to a hatched partin FIG. 9.

As shown in FIG. 9, according to the present embodiment, it wasconfirmed that no gray level inversion occurs in almost all directions,except the 315° azimuth direction. It was also confirmed that even inthe 315° azimuth direction, no gray level inversion occurs in the rangeof the viewing angle of about 50° or less.

FIG. 10 is a view for explaining the characteristics of firstretardation plates RP1 and second retardation plates RP2 which areapplicable to Modifications 1, 2 and 3 of the embodiment, ComparativeExample 1 in which neither the first retardation plate nor secondretardation plate is applied, and the characteristics of a firstretardation plate RP1 and a second retardation plate RP2 of ComparativeExample 2.

In Modification 1 of the embodiment, the first retardation plate RP1 hasa refractive index anisotropy of nx=ny<nz. This first retardation plateRP1 is formed of, for example, a liquid crystal polymer. In the firstretardation plate RP1, the value of the first retardation Re1 is 0 nm,and the value of the first retardation Rth1 is −150 nm. The secondretardation plate RP2 has a refractive index anisotropy of nx>ny>nz.This second retardation plate RP2 is formed of, for example, cycloolefinpolymer. In the second retardation plate RP2, the value of the secondretardation Re2 is 100 nm, and the value of the second retardation Rth2is 90 nm. The optical axis O in the plane of the second retardationplate RP2 is substantially perpendicular to the second absorption axisA2 of the second polarizer PL2. Thus, as regards the first retardationplate RP1 and second retardation plate RP2, the sum Re(sum) of theretardations Re is 100 nm, and the sum Rth(sum) of the retardations Rthis −60 nm. In addition, the contribution degree CNT2 of the secondretardation Rth2 is 37.5%.

In Modification 2 of the embodiment, the first retardation plate RP1 hasa refractive index anisotropy of nz>nx>ny. This first retardation plateRP1 is formed of, for example, maleimide. In the first retardation plateRP1, the value of the first retardation Re1 is 19.8 nm, and the value ofthe first retardation Rth1 is −93 nm. The second retardation plate RP2has a refractive index anisotropy of nx>ny>nz. This second retardationplate RP2 is formed of, for example, cycloolefin polymer. In the secondretardation plate RP2, the value of the second retardation Re2 is 101.5nm, and the value of the second retardation Rth2 is 69.3 nm. The opticalaxis O in the plane of the second retardation plate RP2 is substantiallyperpendicular to the second absorption axis A2 of the second polarizerPL2. Thus, as regards the first retardation plate RP1 and secondretardation plate RP2, the sum Re(sum) of the retardations Re is 121.3nm, and the sum Rth(sum) of the retardations Rth is −23.7 nm. Inaddition, the contribution degree CNT2 of the second retardation Rth2 is42.7%.

In Modification 3 of the embodiment, the first retardation plate RP1 hasa refractive index anisotropy of nx>ny>nz. This first retardation plateRP1 is formed of, for example, cycloolefin polymer. In the firstretardation plate RP1, the value of the first retardation Re1 is 76.3nm, and the value of the first retardation Rth1 is 66 nm. The opticalaxis O in the plane of the first retardation plate RP1 is substantiallyperpendicular to the second absorption axis A2 of the second polarizerPL2. The second retardation plate RP2 has a refractive index anisotropyof nz>nx>ny. This second retardation plate RP2 is formed of, forexample, maleimide. In the second retardation plate RP2, the value ofthe second retardation Re2 is 55.3 nm, and the value of the secondretardation Rth2 is −88.4 nm. Thus, as regards the first retardationplate RP1 and second retardation plate RP2, the sum Re(sum) of theretardations Re is 131.6 nm, and the sum Rth(sum) of the retardationsRth is −22.4 nm. In Modification 3, the retardation plate having apositive retardation Rth, or the retardation plate having a refractiveindex anisotropy of nx>ny>nz, is the first retardation plate RP1.Accordingly, in Modification 3, the contribution degree CNT2 of thefirst retardation Rth1, if calculated from the above-described equation,is 42.7%.

As regards Modifications 1 to 3 of the present embodiment, the viewingangle characteristics of the contrast ratio and the viewing anglecharacteristics of the gray level inversion were simulated, and the sameresults as shown in FIG. 8 and FIG. 9 were obtained.

Comparative Example 1 corresponds to the structure in which neither thefirst retardation plate nor second retardation plate of the presentembodiment is provided. Thus, each of the sum Re(sum) of retardations Reand the sum Rth(sum) of retardations Rth is zero.

In Comparative Example 2, the first retardation plate RP1 has arefractive index anisotropy of nx=ny<nz. In the first retardation plateRP1, the value of the first retardation Re1 is 0 nm, and the value ofthe first retardation Rth1 is −170 nm. The second retardation plate RP2has a refractive index anisotropy of nx>ny>nz. In the second retardationplate RP2, the value of the second retardation Ret is 90 nm, and thevalue of the second retardation Rth2 is 99 nm. Thus, as regards thefirst retardation plate RP1 and second retardation plate RP2, the sumRe(sum) of the retardations Re is 90 nm, and the sum Rth(sum) of theretardations Rth is −71 nm. In addition, the contribution degree CNT2 ofthe second retardation Rth2 is 36.8%.

FIG. 11 is a view showing a simulation result of the viewing anglecharacteristics of the contrast ratio in the liquid crystal displaydevice of Comparative Example 1.

As regards regions where the contrast ratio is 100:1 or more, thefollowing ranges of viewing angles were merely obtained: about 60° at 0°azimuth, about 30° at 45° azimuth, about 80° at 90° azimuth, about 35°at 135° azimuth, about 70° at 180° azimuth, about 35° at 225° azimuth,about 80° at 270° azimuth, and about 35° at 315° azimuth.

In addition, as regards regions where the contrast ratio is 10:1 ormore, the ranges of viewing angles of about 60° were merely obtained at45° azimuth, 135° azimuth, 225° azimuth, and 315° azimuth.

FIG. 12 is a view showing a simulation result of the viewing anglecharacteristics of the gray level inversion in the liquid crystaldisplay device of Comparative Example 1.

As shown in FIG. 12, it was confirmed that gray level inversion occursat 45° azimuth, 225° azimuth, and 315° azimuth. In particular, it wasalso confirmed that gray level inversion occurs in the range of theviewing angle of about 30° or more at 45° azimuth, gray level inversionoccurs in the range of the viewing angle of about 60° or more at 225°azimuth, and gray level inversion occurs in the range of the viewingangle of about 45° or more at 315° azimuth.

It was thus confirmed that Comparative Example 1 is inferior to thepresent embodiment with respect to both the viewing anglecharacteristics of the contrast ratio and the viewing anglecharacteristics of the gray level inversion.

In the present embodiment and Comparative Example 1, the viewing anglevariation of chromaticity was simulated. In this case, the variation ofchromaticity was simulated when the viewing angle was increased from thedirection parallel to the normal line of the liquid crystal displaypanel toward 45° azimuth, 135° azimuth, 225° azimuth, and 315° azimuth.According to simulation results in cases where black (L=0) of the lowestgray level of 256 gray levels was displayed, white (L=225) of thehighest gray level was displayed, and an intermediate gray level (L=127)was displayed, it was confirmed that the variation of chromaticity issmaller in the present embodiment than in Comparative Example 1 in allazimuth directions.

FIG. 13 is a view showing a simulation result of the viewing anglecharacteristics of the contrast ratio in the liquid crystal displaydevice of Comparative Example 2.

According to Comparative Example 2, it was confirmed that the contractratio (CR) is 10:1 or more in all azimuth directions. In addition, asregards regions where the contrast ratio is 100:1 or more, it wasconfirmed that the following ranges of viewing angles can be obtained:about 60° at 0° azimuth, about 40° at 45° azimuth, about 80° at 90°azimuth, about 45° at 135° azimuth, about 75° at 180° azimuth, about 80°at 225° azimuth, about 80° at 270° azimuth, and about 60° at 315°azimuth.

FIG. 14 is a view showing a simulation result of the viewing anglecharacteristics of the gray level inversion in the liquid crystaldisplay device of Comparative Example 2.

As shown in FIG. 14, it was confirmed that gray level inversion occursat 45° azimuth and 315° azimuth. In particular, it was confirmed thatgray level inversion occurs in the range of the viewing angle of about30° or more at 45° azimuth, and gray level inversion occurs in the rangeof the viewing angle of about 60° or more at 315° azimuth. It was alsoconfirmed that gray level inversion occurs in the range of about 30° toabout 50° at 0° azimuth.

It was thus confirmed that in Comparative Example 2, compared to thepresent embodiment, substantially equal characteristics can be obtainedwith respect to the viewing angle characteristics of the contrast ratio,but Comparative Example 2 is inferior to the embodiment with respect tothe viewing angle characteristics of the gray level inversion.

As has been described above, according to the present embodiment, aliquid crystal display device with a good display quality can beprovided.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. A liquid crystal display device comprising: afirst substrate comprising a first insulative substrate, a commonelectrode disposed on an inner surface side of the first insulativesubstrate, an insulation film covering the common electrode, a pixelelectrode which is disposed above the insulation film, opposed to thecommon electrode and provided with a slit, and a first alignment filmcovering the pixel electrode; a second substrate comprising a secondinsulative substrate and a second alignment film disposed on an innersurface side of the second insulative substrate, the inner surface sideof the second insulative substrate being opposed to the pixel electrode;a liquid crystal layer held between the first substrate and the secondsubstrate; a first polarizer disposed on an outer surface side of thefirst insulative substrate; a second polarizer disposed on an outersurface side of the second insulative substrate; and a first retardationplate and a second retardation plate, which are stacked between thesecond insulative substrate and the second polarizer, wherein, withrespect to the first retardation plate and the second retardation plate,when refractive indices in two directions, which are perpendicular toeach other in a plane of each of the first retardation plate and thesecond retardation plate, are nx and ny, respectively, a refractiveindex in a thickness direction is nz, a thickness is d, and aretardation in the thickness direction, which is defined by((nx+ny)/2−nz)*d, is Rth, the first retardation plate has a negativefirst retardation Rth1, the second retardation plate has a positivesecond retardation Rth2, a sum of the first retardation Rth1 and thesecond retardation Rth2 is −40 nm±20 nm with respect to light with awavelength of 550 nm, and a contribution degree of the secondretardation Rth2, which is defined by |Rth2|*100/(|Rth1|+|Rth2|), is40%±3% wherein the second retardation plate has a refractive indexanisotropy of nx>ny>nz.
 2. The liquid crystal display device of claim 1,wherein the first retardation plate has one of a refractive indexanisotropy of nx=ny<nz and a refractive index anisotropy of nz>nx>ny. 3.The liquid crystal display device of claim 2, wherein when an in-planeretardation which is defined by (nx−ny)*d is Re, the first retardationplate has a retardation Re1, and the second retardation plate has aretardation Re2, a sum of the retardation Re1 of the first retardationplate and the retardation Re2 of the second retardation plate is 115nm±15 nm with respect to light with a wavelength of 550 nm.
 4. Theliquid crystal display device of claim 2, wherein the first retardationplate is formed of a material which is different from a material of thesecond retardation plate.
 5. The liquid crystal display device of claim2, wherein the first retardation plate has wavelength dispersioncharacteristics which are different from wavelength dispersioncharacteristics of the second retardation plate.
 6. The liquid crystaldisplay device of claim 1, wherein a first alignment treatment directionof the first alignment film is parallel to, and opposite to, a secondalignment treatment direction of the second alignment film, the firstpolarizer has a first absorption axis which is substantially parallel tothe first alignment treatment direction, and the second polarizer has asecond absorption axis which is substantially perpendicular to the firstalignment treatment direction.
 7. The liquid crystal display device ofclaim 6, wherein an optical axis in the plane of the second retardationplate is substantially perpendicular to the second absorption axis. 8.The liquid crystal display device of claim 6, wherein the slit extendsin a linear shape substantially in parallel to a direction which isdisplaced by 5° to 10° relative to the first alignment treatmentdirection.
 9. The liquid crystal display device of claim 1, wherein thefirst substrate further comprises a source line extending in a directionsubstantially parallel to the slit, and a gate line extending in adirection substantially perpendicular to the source line.
 10. The liquidcrystal display device of claim 1, wherein in a state in which anelectric field is produced between the pixel electrode and the commonelectrode, the liquid crystal layer has a retardation which correspondsto a ½ wavelength.
 11. The liquid crystal display device of claim 1,wherein in a state in which no electric field is produced between thepixel electrode and the common electrode, the liquid crystal layer has aretardation of 300 nm to 350 nm with respect to light with a wavelengthof 550 nm.
 12. A liquid crystal display device comprising: a firstsubstrate comprising a first insulative substrate, a common electrodedisposed on an inner surface side of the first insulative substrate, aninsulation film covering the common electrode, a pixel electrode whichis disposed above the insulation film, opposed to the common electrodeand provided with a slit, and a first alignment film covering the pixelelectrode; a second substrate comprising a second insulative substrateand a second alignment film disposed on an inner surface side of thesecond insulative substrate, the inner surface side of the secondinsulative substrate being opposed to the pixel electrode; a liquidcrystal layer held between the first substrate and the second substrate;a first polarizer disposed on an outer surface side of the firstinsulative substrate; a second polarizer disposed on an outer surfaceside of the second insulative substrate; and a first retardation plateand a second retardation plate, which are stacked between the secondinsulative substrate and the second polarizer, wherein, with respect tothe first retardation plate and the second retardation plate, whenrefractive indices in two directions, which are perpendicular to eachother in a plane of each of the first retardation plate and the secondretardation plate, are nx and ny, respectively, a refractive index in athickness direction is nz, and a thickness is d, the first retardationplate has one of a refractive index anisotropy of nx=ny<nz and arefractive index anisotropy of nz>nx>ny, and the second retardationplate has a refractive index anisotropy of nx>ny>nz, when a retardationin the thickness direction, which is defined by ((nx+ny)/2−nz)*d, isRth, the first retardation plate has a first retardation Rth1 and thesecond retardation plate has a second retardation Rth2, and acontribution degree of the second retardation Rth2, which is defined by|Rth2|*100/(|Rth1|+|Rth2|), is 40%±3%, wherein a sum of the firstretardation Rth1 and the second retardation Rth2 is −40 nm±20 nm withrespect to light with a wavelength of 550 nm.
 13. The liquid crystaldisplay device of claim 12, wherein when an in-plane retardation whichis defined by (nx−ny)*d is Re, the first retardation plate has aretardation Re1, and the second retardation plate has a retardation Re2,a sum of the retardation Re1 of the first retardation plate and theretardation Re2 of the second retardation plate is 115 nm±15 nm withrespect to light with a wavelength of 550 nm.
 14. The liquid crystaldisplay device of claim 12, wherein a first alignment treatmentdirection of the first alignment film is parallel to, and opposite to, asecond alignment treatment direction of the second alignment film, thefirst polarizer has a first absorption axis which is substantiallyparallel to the first alignment treatment direction, and the secondpolarizer has a second absorption axis which is substantiallyperpendicular to the first alignment treatment direction.
 15. The liquidcrystal display device of claim 14, wherein an optical axis in the planeof the second retardation plate is substantially perpendicular to thesecond absorption axis.
 16. The liquid crystal display device of claim14, wherein the slit extends in a linear shape substantially in parallelto a direction which is displaced by 5° to 10° relative to the firstalignment treatment direction.